Typically in a two-way radio, a frequency synthesizer is used to generate a receive local oscillator (LO) signal and a transmit carrier. The frequency bands for receive and transmit operation may or may not be the same for the two-way radio. Differences may result from the receiver architecture (e.g., superheterodyne), system design (e.g., separate frequency blocks for transmit and receive signals) or transmitter architecture (e.g., frequency offset). When the radio's transmitter is operative, high levels of radio frequency signals are radiated back into the radio from the antenna. This tends to cause several modes of self-interference of the transmitter.
Two approaches are currently used to meet the requirements of this radio environment. These approaches are the offset loop and the image-balanced mixer. In the offset loop approach, a phase-locked loop (PLL) comprising a phase detector, loop filter, voltage-controlled oscillator (VCO), offset mixer and offset signal source are connected between the synthesizer output and the input of the transmitter gain and linearization path. The offset loop shifts the output frequency of the synthesizer VCO by an amount equal to the offset signal frequency. A wide loop bandwidth in the offset loop provides immunity to the offset loop VCO from transmit energy. The loop also tends to limit spurious responses caused by the presence of the offset signal.
The offset loop approach has the limitation that in order to minimize the spurious response of the offset loop due to unwanted mixing products, the mixer must be operated such that its response is highly linear. This tends to increase power consumption and limits the noise floor of the system. Another limitation of the offset loop approach is that an additional signal must be provided to the system. Generation of the offset signal increases the cost of the design as well as increases the power consumption for the radio. Also, the presence of the offset signal creates an opportunity for a spurious output of the loop that is only mitigated by the action of the PLL.
In the image-balanced mixer approach, the synthesizer output is mixed with an offset signal and no loop is used to remove spurs from the output. This design is limited to use in systems where the transmit frequency range is much smaller than the offset frequency. A multi-pole filter is required to remove the offset and higher order spurs from the mixer output. The filter however adds cost to the design and constrains the operating frequency range of the transmitter. A need thus exists in the art for a means of generating a carrier frequency without use of an offset signal or mixer based use of an offset loop while minimizing noise in the spectrum of the offset signal.